Ink jet recording is a process in which an ink is ejected as small droplets from a minute nozzle to record characters or images on a surface of a recording medium. Techniques of ink jet recording which have been put to practical use include: a method comprising converting electrical signals to mechanical signals with an electrostrictive element and intermittently ejecting an ink stored in a nozzle head to record characters or images on a surface of a recording medium; and a method comprising bubbling an ink stored in a nozzle head by rapidly heating that part of the ink which is located very close to the orifice and intermittently ejecting the ink based on the volume expansion caused by the bubbling to thereby record characters or images on a surface of a recording medium.
Known techniques for supplying inks in ink jet recording include a technique employing an ink set comprising a combination of inks of different colors. In ink jet recording with such an ink set, the kind and amount of the ink to be ejected from the ink set are always selected according to signals based on image information. This ink jet recording technique hence has an advantage that a color image such as, e.g., a photographic image can be easily obtained with high image quality.
An ink set comprising, in terms of one same hue, a plurality of inks having different colorant concentrations (which may be referred to as “deep/light ink set”) is known. In application to an area to be printed so as to have a given printing density, an image having reduced graininess can be obtained by positively increasing the ink duty (applied amount of ink per unit area) for an ink having a low colorant concentration (which may be referred to as “light ink”) in comparison with the ink duty of an ink having a high colorant concentration (which may be referred to as “dark ink”).
However, especially when such an ink set is used in printing an image requiring an increased ink duty (in particular, a photographic image), then the paper (especially plain paper) absorbs the water contained in the inks and expands. The paper is hence apt to become wavy or bent to pose the possibility that the deformed paper might come into contact with the printer head to make the desired printing unavailable. For avoiding this trouble, printers are usually equipped with rollers which hold the paper therebetween to thereby keep it in a given position and preventing it from coming into contact with the head.
In general, as inks of the ink sets to be subjected to such printers, those prepared by dissolving various water-soluble dyes in an aqueous medium are commonly used. Recently, however, inks prepared by dispersing a pigment in an aqueous medium with the aid of a dispersant have also come to be provided. This is because such inks containing a pigment (which may be hereinafter referred to as “pigment inks”) are characterized by being superior to inks containing a water-soluble dye in weatherability (e.g., water resistance and light resistance).
Pigments are generally insoluble in water. Consequently, when a pigment is used as a colorant in preparing an aqueous ink composition, the pigment is mixed with water together with a dispersant to stably disperse it before being formulated into an ink composition.
However, even the pigment ink thus prepared is less apt to penetrate into paper especially when plain paper is used or when an image requiring a high ink duty, such as, e.g., a photographic image, is to be formed. There is a problem that in ouch a case, the ink remaining unpenetrated on the paper is apt to adhere to the rollers and the resultant ink-bearing rollers are apt to stain the image (hereinafter, this phenomenon may be referred to as “ink transfer”).
In the case where the dark/light ink set described above is used to conduct printing in order to reduce the graininess of images, the amount of the inks applied to the paper tends to be large, for example, because the duty of the light-color ink is regulated to be high. As a result, the inks are less apt to penetrate into the recording paper, and the irks remaining unpenetrated cause ink transfer and tend to result in recognizable stain on images.
Such stain on images is apt to be conspicuous especially when an ink set comprising dark and light inks having high pigment concentrations is used so as to obtain an image with high color development.
Furthermore, there is alas a problem that high speed printing, which is recently desired, tends to cause the ink to adhere to rollers before penetrating into the paper, and this may be a cause for accelerating ink transfer.
In addition, use of pigment inks has had a problem that although the inks have excellent weatherability as stated above, image fixation to the paper is insufficient or the recorded matter obtained is apt to have unevenness attributable to a difference in gloss between areas having different ink duties (which may be referred to as “gloss unevenness”).